During the extension of the INTEGRATE project, L’Institut Agro aimed at testing the feasibility of associating for the first time the rearing of sea cucumber Holothuria forskali with the flat oyster, Ostrea edulis in open sea near-shore. The experiment was designed to verify if this association can be a successful IMTA system, meaning that the holothurians benefit from the waste of the oysters. L’Institut Agro also tested different stocking densities of Holothurians to identify the best rearing conditions to maximize growth and survival. To realistically qualify the system an IMTA, trophic links between the two species need to be demonstrated, this was performed by analysing the fatty acid and isotope signatures of the different compartments of the system.

The experimental flat oysters and holothurians were stocked separately in plastic cages of dimensions 87 cm x 45 cm x 12 cm (Figure 1A) stacked in the experimental structures. Below multiple layers of flat oyster cages, different densities of holothurians per cages were tested (Figure 2B) : 8, 16, 24, and 32 individuals per cage bottom square meters. A last condition consisted in stocking the lowest density above the flat oyster. The experiment was ran in triplicated structures with the order of the conditions below the flat oysters changed. Two temperature probes were installed on two structures. The holothurians were 5g juveniles bred in the experimental facilities of L’Institut Agro.

Figure 1. Experimental cages (A) and experimental set up (B) to test growth and survival potential of holothurian juveniles (C) stocked at different densities underneath flat oysters.

The experiment started in March 2022 and ended in May 2023. The water temperature fluctuated the experimental period from 9 to 20.5 °C with a mean around 15°C. A total of 4 biometries were performed after the start of the experiment. Results show excellent survival for all conditions (99%) and exceptional growth of the holothurians with individuals reaching up to 20x their initial weight in 15 months (Figure 3A). Surprisingly, the holothurians stocked above the flat oysters showed a growth very similar to the same stocking condition (8 holothurians.m-2) stocked underneath, with even a better growth at the end of the experiment (Figure 3A). A clear decrease in growth potential was observed with increasing stocking densities underneath the flat oysters, translating into an exponential decrease in the final mean weights (Figure 3B). Yet, the highest densities were found to be still interesting since they show the highest total final weight (Figure 3C).

Figure 2. Growth differences between stocking conditions over time (A), final mean (B) and total (C) per replicated cage for each density below the oyster cages.

Finally, the fatty acids compositions of the holothurians showed clear differences for conditions above and below the oysters (at the lowest stocking densities), which translated into a significant difference on the second dimension of the Principal Composant Analysis ran on all fatty acids (Figure 4). Isotope analyses are still in progress to confirm these results.

Figure 4. Coordinates on the second axis of a Principal Component Analysis performed on the fatty acids profiles of holothurians reared at 3 different conditions. The first two boxplots represents 2 densities of holothurians reared underneath flat oysters, while the last one illustrates results of holothurians reared above oyster cages.

Altogether, this pilot study shows the great potential of using Holothuria forskali to rear in combination with flat oysters. Such a system would at least allow to diversify the production. The biochemical composition analysis (Fatty acids) suggests that food sources are different for individuals stocked underneath the flat oysters compared to individuals above. This provides the first argument to stipulate that H. forskali is able to use oysters’ waste when stocked underneath. If this is confirmed by isotope analyses, this will provide proofs that the pilot study is a successful IMTA, and that H. forskali help to reduce organic waste accumulation under bivalves production.

These results will be presented at the European Aquaculture Society conference in Vienna (2023) during an oral presentation.